This invention relates generally to feedthrough capacitor terminal pin subassemblies and related methods of design and construction, for protecting implantable medical devices from electromagnetic interference commonly found in the environment. More specifically, the present invention relates to improved performance feedthrough capacitor terminal pin subassemblies which offer attenuation to EMI at lower frequencies and also at higher attenuation levels, particularly in medical implant applications.
Feedthrough terminal assemblies are generally well known for connecting electrical signals through the housing or case of an electronic instrument. For example, in an implantable medical device, such as a cardiac pacemaker, defibrillator or the like, the terminal pin assembly comprises of one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are typically connected to one or more lead wires which are connected to cardiac tissue or other tissue to be stimulated which effectively act as an antennae and tend to collect stray electromagnetic interference (EMI) signals for transmission into the interior of the medical device. The hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough filter capacitor to decouple interference signals to the housing of the medical device.
Most implantable medical devices in the United States today incorporate feedthrough capacitor EMI filters at their input terminals or in conjunction with the hermetic terminal. However, due to size constraints and mechanical constraints, the capacitance value of these filters has been relatively low (in the 490 to 4000 picofarad range). These capacitance values have been very effective for attenuation of cell phones and other high frequency emitters, however, they do very little to protect the implantable medical device against lower frequency EMI.
It has been well-documented in in-vivo and in-vitro studies that certain kinds of electromagnetic interference can cause disruption of the implantable medical device. For example, in cardiac pacemakers it has been shown that digitally modulated EMI can cause pacemaker inhibition, asynchronous pacing or missed beats. All of these conditions would be highly undesirable and potentially life threatening in a pacer-dependent patient. It has also been shown that EMI can cause an implantable cardioverter defibrillator to inadvertently deliver its high voltage shock therapy. This is very uncomfortable to the patient and is equivalent to a very hard blow to the chest. In prior art devices, such as those shown in U.S. Pat. Nos. 5,333,095; 4,424,551; 5,905,627; 5,751,539 and 6,008,980 (the contents of which are incorporated herein), the hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough capacitor filter to decouple and shield electromagnetic interference (EMI) signals into the housing of the medical device.
For example, FIG. 1 is a cut away view of a typical cardiac pacemaker 30 showing an internal circuit board 32 and a broadband EMI filter 34. In order for the broadband EMI filter 34 to work properly, it must be mounted directly at the point of lead 36 ingress and egress.
The broadband EMI filer 34 is typically of coaxial construction also known as a feedthrough capacitor EMI filter. The feedthrough capacitor 34 is optimally bonded directly to the hermetic terminal 38 (FIG. 2) of the implantable medical device that is used to exclude entry of body fluid. The location of the broadband EMI filter 34 at the point of lead ingress and egress is essential so that undesirable incoming EMI signals can be decoupled and shunted directly to the titanium or stainless steel pacer or can or housing 40 and dissipated as harmless energy (heat).
With reference to FIG. 2, in a typical prior art unipolar construction (as described in U.S. Pat. No. 5,333,095), a round/discoidal (or rectangular) ceramic feedthrough filter capacitor 42 is combined with a hermetic terminal pin assembly 38 to suppress and decouple undesired interference or noise transmission along a terminal pin or lead 36. The feedthrough capacitor 42 is coaxial having two sets of electrode plates 44, 46 embedded in spaced relation within an insulative dielectric substrate or base 48, formed typically as a ceramic monolithic structure. The dielectric substrate or base 48 is generally constructed of barium titinate dielectrics that have been built doped with suitable materials to form the desired dielectric properties. One set of the electrode plates 44 is electrically connected at an inner diameter cylindrical surface of the coaxial capacitor structure 42 to the conductive terminal pin 36 utilized to pass the desired electrical signal or signals. The other or second set of electrode plates 46 is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule 50 of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing 40 of the electronic device 30. The number and dielectric thickness spacing of the electrode plate sets 44, 46 varies in accordance with the capacitance value and the voltage rating of the coaxial capacitor 42. The outer feedthrough capacitor electrode plates 46 (or xe2x80x9cgroundxe2x80x9d plates) are coupled in parallel together by a metallized layer 52 which is either fired, sputtered or plated onto the ceramic capacitor 42. This metallized band, in turn, is coupled to the ferrule 50 by conductive adhesive, soldering, brazing, welding, or the like. The inner feedthrough capacitor electrode plates 44 (or xe2x80x9cactivexe2x80x9d plates) are coupled in parallel together by a metallized layer 54 which is either glass frit fired or plated onto the ceramic capacitor 42. This metallized band 54, in turn, is mechanically and electrically coupled to the lead wire 36 by conductive adhesive or soldering, or the like. In operation, the coaxial capacitor 42 permits passage of relatively low frequency electrical signals along the terminal pin 36, while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the conductive housing 40. Feedthrough capacitors of this general type are available in unipolar (one), bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (6) and additional lead configurations. The feedthrough capacitors (in both discoidal and rectangular configurations) of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing 40 is constructed from a biocompatible metal such as titanium alloy, which is electrically and mechanically coupled to the hermetic terminal pin assembly 38 which is in turn electrically coupled to the coaxial feedthrough filter capacitor 42. Alternatively, the feedthrough capacitor can be grounded to one or more terminal pins as described in U.S. Pat. No. 5,905,627. As a result, the filter capacitor 42 and terminal pin assembly 38 prevents entrance of high frequency interference signals to the interior of the pacemaker housing 40, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
Feedthrough filter capacitors for cardiac pacemakers and the like, have typically been constructed by preassembly of the coaxial capacitor 42 onto or within a cylindrical or rectangular hermetically sealed terminal pin subassembly 38 which includes the conductive pin 36 and ferrule 50. More specifically, the terminal pin subassembly 38 is prefabricated to include one or more conductive terminal pins supported within the conductive ferrule by means of a hermetically sealed insulator ring or bead 56. One type of hermetic terminal pin subassembly 38 which is widely used in implantable medical devices employs an alumina ceramic insulator 56 which is gold brazed into a titanium ferrule 50. In addition, the platinum lead wire 36 is also gold brazed to the alumina ceramic insulator 56 to complete the hermetic seal. See for example, the subassemblies disclosed in U.S. Pat. Nos. 3,920,888; 4,152,540; 4,421,947; and 4,424,551. An improved design in the prior art which has substantially improved the volumetric efficiency is based upon surface mounting of a ceramic feedthrough capacitor planar array structure to one outer surface of a hermetic terminal with similar connection to the conductive pins (see the subassemblies disclosed in U.S. Pat. No. 5,333,095).
Prior art feedthrough capacitors used as EMI filters in implantable medical devices have all been limited in capacitance value. This is due to the extreme constraints put on volume, size and weight in active implantable medical devices. Major trends affecting the implantable medical electronics industry are the following factors:
1. Reduction in size.
2. Reduction in weight.
3. Increase in longevity or battery life.
4. Increase in number of functions.
5. Reduction in cost.
6. Increase in performance.
Prior art feedthrough capacitors have generally used commercial (EIA Designations) dielectrics designated by BX, X7R or NPO types. These are barium titinate dielectrics that grew out of use in military, space applications and other mission critical applications.
The Electronics Industry Association (EIA) has developed a number of codes for capacitors known as EIA Standard RS198. Included in this standard are a number of descriptive letter codes (Z5U, Y5V, X7R, etc.) that designate how much the capacitance of a capacitor will change over various temperature ranges. The first letter signifies the low operating temperature. The second letter indicates the maximum amount the capacitance will change. The number between gives the high operating temperature. FIG. 3 gives a few examples of how this code works. In particular, FIG. 3 illustrates prior art EIA temperature characteristic codes that are commonly used in monolithic ceramic capacitor industry. This explains how the codes work. We will use an X7R capacitor as an example. The letter X designates the low temperature operating point which you can see from the Table is xe2x88x9255xc2x0 C. The xe2x80x9c7xe2x80x9d illustrates the high temperature operating point which is 125xc2x0 C. The xe2x80x9cRxe2x80x9d designates the maximum allowable capacitance change between these two temperature extremes, which in this case is +/xe2x88x9215%. As a second example, we could consider Y5V. xe2x80x9cYxe2x80x9d would indicate that the low temperature operating point is xe2x88x9230xc2x0 C. xe2x80x9c5xe2x80x9d indicates the high temperature operation point is 85xc2x0 C. The xe2x80x9cVxe2x80x9d indicates that the capacitance can change in value over that full range any where from +22% to xe2x88x9282% (a total chnage in capacitance of 104%).
These dielectrics are generally formulated starting with pure barium titinate, which inherently has a very high dielectric constant (K). Unfortunately, pure barium titinate also is very unstable in terms of temperature and voltage. Most titinates (for instance, barium titinate) undergo crystalline inversions which result in very high values of dielectric constant near the temperature at which the shift occurs (the Curie point). In the case of barium titinate, the Curie temperature is about 120xc2x0 C. At temperatures above the Curie temperature, titinates are not ferro-electric (they are paraelectric). At temperatures below the Curie temperature, they are ferro-electric and can be used in piezoelectric applications. The alkaline earth titinates (calcium, strontium, magnesium and barium) are the basic materials used in making monolithic ceramic capacitors. The most commonly used is barium titinate. As previously mentioned, it has a Curie point at about 120xc2x0 C.
FIG. 4 illustrates the change in dielectric constant (or K) with temperature between 25xc2x0 C. and 120xc2x0 C. of pure barium titinate. Materials can be added to the high K barium titinate ceramic to depress the Curie peak, resulting in a formulation that has less temperature dependency. One of the trade-offs is that this tends to lower the dielectric constant. Increasing the amount of depressor decreases the amount of temperature variability. FIG. 5 illustrates a prior art technique of using a depressor to depress the Curie point peak. This has the effect of making the dielectric constant of the capacitor much more stable over the operating temperature range. Materials used to depress the Curie peak usually stay in the grain boundary of the polycrystalline ceramic as opposed to Curie shifters which enter into the crystalline structure itself. As previously shown in FIG. 4, unmodified barium titinate has a very high dielectric constant at its Curie temperature, which drops to relatively low values at a low temperature, such as room temperature at 25xc2x0 C. This relates directly to the volumetric efficiency of the finished capacitor. For example, a capacitor built from pure barium titinate would have a dielectric constant of roughly 1800 at 25xc2x0 C., but over 22,000 at 120xc2x0 C. This capacitor would have tremendous volumetric efficiency if operated at only 120xc2x0 C. Referring now again to FIG. 5, depressors are added generally into the grain boundary of the polycrystalline ceramic. This is different from Curie point shifters, which generally enter into the crystalline structure itself. Depressors mean that the capacitor will be made more stable over temperature. However, the trade off is the sacrifice of dielectric constant. In the extreme case, the dielectric constant can be depressed all the way down to below 100 and that means that its variation in dielectric constant and also capacitance will be less than 0.5% when the device is exercised over the temperature range xe2x88x9255 to +125xc2x0 C.
Curie point shifters are materials which can be added to barium titinate that will shift the Curie temperature to another temperature. For example, lead titinate is one material that will move the Curie temperature up. Strontium titinate is an example of a material that will shift it downward. Shifters enter into solid-state solution with the barium titinate. The resulting misfit in the crystal lattice causes energy changes which makes the crystalline inversion occur at a different temperature. FIG. 6 illustrates the concept of using prior art Curie point shifters. Shifters are materials that are added to the barium titinate that will shift the Curie temperature to another temperature. Lead titinate will generally move the Curie temperature up. Strontium titinate and other materials are examples of materials that will shift it downward.
Shifters enter into solid-state solution with the barium titinate. A resulting misfit in the crystal lattice causes energy changes which makes the crystalline inversion occur at a different temperature. FIG. 6 illustrates curve 1 which is pure barium titinate with an inherent Curie point of roughly 120xc2x0 C. Curve 2 indicates just enough shifter to shift the Curie point to 37xc2x0 C., which is body temperature. This has not been done before in a standard commercial dielectric, but would optimize the device for operation only at 37xc2x0 C. (body temperature). Curve 3 illustrates a curve with the addition of more shifter which would cause the Curie peak to occur at roughly room temperature (20xc2x0-25xc2x0 C.). There are a set of commercial dielectrics that are commonly available that are represented by FIG. 3, including Y5V and other commercially available dielectrics.
In the past, such unstable dielectric materials such as Y5V have not been used for human implant applications because they are generally considered unreliable and are too temperature unstable (at 37 degrees C. there is still about a 20 to 40 percent drop in capacitance compared to operation right at the 20 to 25xc2x0 C. Curie peak of Y5V). For general military and space applications, ceramic dielectrics have been formulated which are very temperature stable over the full operating military and space temperature ranges from xe2x88x9255xc2x0 to +125xc2x0 C. Accordingly, the inherent dielectric constant of barium titinate, which can run as high as 22,000, has been depressed into the range of approximately 2500 or even lower. Volumetric efficiency is sacrificed for this depression of Curie point; however, the change in capacitance with temperature can be held to very tightly controlled limits. For example, in the case of BX and X7R, the maximum allowable temperature change is +/xe2x88x9215% over the operating temperature range from xe2x88x9255 to +55xc2x0 C., which is acceptable for most military, space and other mission critical applications.
Another very important aspect to consider with ceramic dielectrics is voltage stability. When DC bias voltage is applied to a monolithic ceramic capacitor, the amount of capacitance value drops. The amount of drop is related to the volts/mil stress on the ceramic dielectric and also the dielectric constant itself. The volts/mil stress is simply found by taking the dielectric thickness between the electrodes of the capacitor and dividing that thickness into the applied voltage. For example, a capacitor with a 100 volts applied to it that has 2 mils of fired dielectric thickness would have an applied stress of 50 volts/mil. When 50 volts/mil is applied to a ceramic BX dielectric with a dielectric constant of around 1500 (this means the Curie point has been significantly depressed) the capacitance will drop approximately 10% in value as illustrated in FIG. 7. A drop of 10% is generally acceptable in military, space and related applications. Also, by doubling the dielectric thickness, the stress becomes 25 volts/mil which one can see from FIG. 7 only causes a drop of approximately 3.5% in capacitance (and even more drop in volumetric efficiency).
More specifically, FIG. 7 illustrates the propensity of a prior art 1500K and BX monolithic ceramic capacitor to drop slightly in capacitance with applied DC bias voltage. What this means is that the dielectric stress between opposed electrodes causes the crystal lattice to drop in K. The way one interprets FIG. 7 would be best illustrated by an example: Let""s assume that we have designed a capacitor with a dielectric thickness of 2 mils after all manufacturing operations including sintering. If one were to apply 100 volts DC bias to this capacitor, one could readily calculate the DC voltage stress in volts/mil. We would simply take the 100 volts applied and divide it by 2 mils which would yield a stress on the dielectric of 50 volts/mil. As one can see, at 50 volts/mil the capacitance is down approximately 10%. This is typical of a commercial designation BX dielectric, which in general has a K of roughly 1500 to 2800. This device has been quite popular in the prior art and is used in many high reliability applications in military, space and medical applications.
FIG. 8 illustrates the temperature coefficient (TC) curve of the same dielectric as described in FIG. 7. In this case, it has an inherent dielectric constant of 1500. This illustrates the change in capacitance with temperature. As can be seen, this is a relatively stable dielectric with a Curie point of approximately 120xc2x0 C. At xe2x88x9255xc2x0 C., the capacitance drops about 9% and at +125xc2x0 C., the capacitance drops approximately 7%. This is a very stable dielectric suitable for use in military and space applications. The negative trade-off is that it is not very volumetrically efficient.
The foregoing discussion relates to representative examples of typical dielectrics found in the prior art. These curves were published by Solid State Dielectrics, Inc., which initially formulated these materials. FIG. 8 illustrates the change in capacitance with temperature of the 1500 K dielectric previously described. As one raises the dielectric constant, the capacitor becomes more volumetrically efficient. However, both the temperature stability and voltage stability of the capacitor are severely affected. FIG. 9 illustrates the temperature stability of a common commercial dielectric known as Z5U. It has a dielectric constant of around 7000 and has a much greater change in capacitance over temperature stability. As one can see, the operating temperature range is narrowed. For a typical military-type capacitor, the operating temperature range would be specified for xe2x88x9255 to +125xc2x0 C. However, the Z5U dielectric is only rated to operate between xe2x88x9225 and +85xc2x0 C. Over this restricted operating temperature range, the capacitance drops as much as 50% in capacitance from its initial room temperature value.
The application of DC voltage also greatly affects this higher K dielectric. FIG. 10 illustrates the change in capacitance with applied DC voltage in volts/mil. Assuming the same example that we previously described for the BX dielectric, as one can see, at 50 volts/mil, this dielectric drops almost 80% in capacitance. It also should be noted that the capacitance change and temperature change effects are accumulative. That is if this capacitor was operated at 125xc2x0 C. with 50 volts/ml bias, one would only have a few percent of the initial capacitance left. This is one reason why this particular grade of capacitor dielectric is generally not used in high reliability/mission critical applications. It is a common component in consumer electronics, such as car stereos and the like, which are generally only expected to operate at low voltage and near room temperature.
The effects of voltage bias and temperature are cumulative. For example, if one were to take this Z5U commercial dielectric and cool it down to xe2x88x9225xc2x0 C. while at the same time applying 50 volts/mil, almost all of the capacitance would be gone.
Another prior art commercial dielectric is known as Y5V. It has a dielectric constant above 12,000 and is very volumetrically efficient for room temperature applications. FIG. 11 illustrates the change in capacitance with temperature, which is even more severe than the previously described Z5U. For example, at xe2x88x9225xc2x0 C., this particular dielectric loses 80% of its capacitance. The higher the dielectric constant is raised, the more unstable the capacitor becomes both with applied temperature and applied DC bias. At xe2x88x9225xc2x0 C., the capacitance drops approximately 80% and at +85xc2x0 C., the capacitance drops about 82%.
Y5V is also remarkably unstable in the presence of applied DC voltage. FIG. 12 represents the prior art Y5V capacitor""s drop in capacitance with applied DC bias. Using the same example of a capacitor of 2 mils of fired dielectric thickness and 100 volts DC applied, one has a dielectric stress of 50 volts/mil. At this 50 volts/mil stress level from the FIGURE, one can see that there is an approximate 90% drop in capacitance. This normally would be highly undesirable in an EMI filter because greatly reduced attenuation would result. While the use of this commercial dielectric might be contemplated at 37 degrees (human implant temperature), as one can see from FIG. 11, the capacitance value will drop almost 40 degrees C. as compared to the Curie peak which is between 20 and 25 degrees C.
As one increases the dielectric constant, one also gives up a number of other design parameters in addition to temperature stability and voltage sensitivity. One of these is dielectric breakdown strength. In general, as one increases the dielectric constant of the capacitor, the dielectric breakdown strength measured in volts/mil declines. FIG. 13 generally illustrates this effect showing that as one increases the dielectric constant to above 10,000, the dielectric breakdown strength declines significantly. Specifically, FIG. 13 represents prior art ceramic dielectrics. The X-axis shows the dielectric constant and the Y-axis indicates the dielectric breakdown strength between the electrode plates measured in volts DC/mil. As one can see, in general, dielectric strength tends to drop the higher the dielectric constant. For example, low K dielectrics (less than 100 K) generally have dielectric breakdown strengths of over 1000 volts/mil. However, high K commercial dielectrics (that are in the range of 7000 or higher) generally have dielectric breakdown strengths just slightly above 400 volts/mil. It should be noted that as you increase the K, mechanical strength is also sacrificed.
Ferro-electric materials exhibit a number of special properties, including dielectric historisis. Ferro-electric materials are also piezoelectric, in that they have capabilities of converting mechanical movement or pressure to electrical signals or energy and visa versa. Barium titinate is generally known as a ferro-electric material when operating below its Curie point. The capacitor""s ferro electric or piezoelectric behavior is a major design consideration for EMI feedthrough capacitors used in the output of implantable cardioverter defibrillators. When a high voltage pulse is applied to the EMI filter, significant mechanical stresses are generated. One of the reasons for the great commercial success of U.S. Pat. Nos. 5,333,095 and 5,905,627 is that these designs allow the ceramic capacitor to expand and contract during the application of the high voltage pulse. In this way, the piezoelectric stresses are well managed.
Another important design property that one sacrifices as the dielectric constant increases is mechanical strength. High K dielectrics simply are not as mechanically strong both in terms of tensile, compression or breaking strength as compared to lower K dielectrics. In addition, they exhibit a much lower modulus of toughness. It is a general principle of ceramic engineering that low K dielectrics are mechanically very robust. An example of this is the alumina (AL2O3) insulator used in the hermetic insulators of implantable medical devices. Alumina insulators have excellent mechanical properties including a very high modulus of toughness and high tensile and compressive strength. Accordingly, they make for very robust hermetic seals through which lead wires pass. They can withstand the heat of laser welding and also handling by operators during the manufacturing and installation of the device. Alumina (Al2O3) ceramic, however, has a very low dielectric constant (less than 7) and could not be used to make a feedthrough capacitor due to the very poor volumetric efficiency that would result.
For the reasons described above, extremely high K dielectrics (above K=7000) have not been used extensively in high reliability and/or high performance military and space applications. In military and space applications, where temperature stability and voltage stability are critical, such materials are contraindicated. High K dielectrics such as Y5V and Z5U do find commercial applications in consumer electronics, computers and the like. The reason for this is that such devices are not exposed to extremes in temperature conditions and the voltage applied to the capacitors can be managed to be very stable. There is no known application where a high K dielectric (above 7000) has been used for the feedthrough capacitor EMI filter of an implantable medical device.
High K dielectrics have also been contraindicated in mission critical applications due to the general impression in the industry that they are unreliable. By unreliable, it is meant prone to failure, either due to degradation of insulation resistance, or outright electrical short (catastrophic failure).
Accordingly, there is a need for a highly reliable monolithic ceramic feedthrough capacitor which is utilized in combination with the hermetic seals of active implantable medical devices such as cardiac pacemakers, implantable cardioverter defibrillators (ICDs), cochlear implants and the like, wherein the feedthrough capacitor dielectric material comprises a very high dielectric constant material whose Curie point has been shifted to approximately 37 degrees C. with very little or no Curie peak depressor. Such a feedthrough capacitor would advantageously push the effective K available to above 7000 and allow EMI feedthrough capacitors for implantable medical devices to be built in the same physical size or smaller with much higher capacitance values. The present invention fulfills these needs and provides other related advantages.
The present invention describes monolithic ceramic feedthrough capacitors used in combination with the hermetic seals of active implantable medical devices such as cardiac pacemakers, implantable cardioverter defibrillators (ICDs), cochlear implants, neurostimulators, and the like. The feedthrough capacitor dielectric is a novel very high dielectric constant material whose Curie point has been shifted to approximately 37 degrees C. with very little to no Curie peak depressor. This pushes the effective K available to above 7000 (up to a maximum of approximately 22,000). This allows EMI feedthrough capacitors for implantable medical devices to be built in the same physical size (or even smaller) with much higher capacitance values. The higher capacitance provides additional attenuation to EMI caused by emitters in the frequency range from the high kilohertz all the way up to several gigahertz.
The present invention embraces a dielectric technology wherein the capacitance value of the EMI filter can be raised to much higher levels therefore providing immunity to EMI over a much broader frequency range.
A particular feature of the present invention is a unique property of the high dielectric constant material to decrease in capacitance with applied voltage. This is of particular advantage in output of an implantable cardioverter defibrillator (ICD). Excessive capacitance on the output leads of the ICD can degrade the pulse and also affect its energy efficiency. It is a property of high K dielectric capacitors that they drop drastically in capacitance value during the application of high voltage thereby preventing this problem. During the quiescent period, in other words, when the implantable defibrillator is not providing high voltage shock therapy, the full capacitance value is available to protect against electromagnetic interference. This is important as most EMI problems occur when the implantable device is in the cardiac signal sensing mode and detecting signals form hundreds of microvolts to several millivolts.
More particularly, the present invention relates to a novel process for manufacturing a monolithic ceramic capacitor for use in an active implantable medical device. The process steps include providing a dielectric material having a dielectric constant greater than 7000, adding one or more Curie point shifters to the dielectric material to optimize the monolithic ceramic capacitor dielectric constant at the human body temperature of 37xc2x0 C., and processing the dielectric material with one or more shifters into a finished capacitor having first and second sets of electrode plates disposed therein. In the preferred embodiment, a dopant may be used to broaden the Curie point peak of the dielectric material, and the monolithic ceramic capacitor is typically configured as a feedthrough filter capacitor designed for incorporation into an EMI filter. Such a filter is useful when installed in a high voltage defibrillator circuit of an implantable medical device. The dielectric material is further optimized so that during the delivery of high-voltage electrical energy, such as a bi-phasic or mono-phasic fast rise time defibrillation pulse, the capacitance value of the capacitor drops to less than 65% of its initial value, and preferably less than 20% of its initial value.
In a feedthrough filter capacitor assembly for use in an active implantable medical device, the invention comprises at least one conductive terminal pin, the novel feedthrough filter capacitor of the present invention, and a conductive ferrule through which the terminal pin passes in nonconductive relation. The feedthrough filter capacitor has first and second sets of electrode plates disposed within a dielectric material having a dielectric constant greater than 7000, and a first passageway through which the terminal pin extends in conductive relation with the first set of electrode plates. The feedthrough filter capacitor is optimized for operation within the temperature range of 30xc2x0 C. to 40xc2x0 C., and preferably at 37xc2x0 C. In a preferred embodiment, the dielectric constant of the dielectric material is within the range of 8500 to 22,000. The dielectric material includes at least one dopant used to shift the Curie point to 37xc2x0 C. It is often necessary to include a Curie point depressor for broadening the temperature bandwidth of the Curie point peak or point of maximum dielectric constant. Typical dopants utilized include strontium titinate, lead titinate, zirconium dioxide, barium carbonate, zinc oxide, zinc borate, manganese carbonate, and titanium dioxide. Typical applications include cardiac pacemakers, ICD""s, hearing implants, congestive heart failure treatment devices, atrial defibrillators, bi-ventricular pacemakers, neurostimulators, brain stimulators, bladder control stimulators, artificial eyes, artificial noses, RF muscle actuators, implanted limb manipulation systems, artificial hearts and ventricular assist devices.
The present invention is further particularly useful in feedthrough filter capacitor assemblies for use in human implant applications utilizing an internal ground configuration. There the assembly includes at least one conductive terminal pin, a conductive ferrule through which the terminal pin passes in non-conductive relation, the novel feedthrough filter capacitor of the present invention, and a ground lead which extends into a second passageway through the feedthrough filter capacitor, wherein the ground lead is conductively coupled to the second set of electrode plates and the conductive ferrule.
Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.